1. Field of the Invention
The present invention relates generally to fluid level detectors, more particularly to out-of-fluid sensors for hard copy machines, and, more specifically, to an out-of-ink detector for an ink-jet pen.
2. Description of the Related Art
A universal problem in hard copy technology is the fact that ink supplies are not unlimited. The same applies for those types of hard copy machines that use wet or dry toners, dyes, or other colorants from a reservoir to produce a hard copy. As a result, at some point during an operational cycle, the machine will run out. This problem can be particularly egregious in ink-jet technology where a replaceable ink reservoir is employed.
The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39. No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions, incorporated herein by reference.
Running an ink-jet pen dry can damage the printhead mechanism. Even if the printhead mechanism is not damaged, the pen will have to be re-primed once the reservoir is refilled or replaced. This requires extra hardware, generally in the form of a pen service station, to be included in the printer mechanism. If a pen runs low on ink, the hard copy equipment user is also inconvenienced by poor print quality or an unusable partial print which, particularly in complex, large scale, engineering drawings, can be costly.
Some original equipment manufacturers have provided machine readable ink level sensors. Generally, these sensors rely on electrical properties of ink and foam reservoir material interactions within the pen. Accuracy of such devices is questionable. Such sensors are inapplicable to off-board ink reservoir systems.
Early ink-jet printers used off-pen reservoirs as demonstrated in U.S. Pat. No. 4,312,007. Moreover, it has been found that for some hard copy applications, such as large format plotting of engineering drawings and the like, there is a requirement for the use of much larger volumes of ink than can be contained within replaceable pens that have on-board ink reservoirs. Therefore, relatively large, off-pen, ink reservoir systems have also been developed more recently. As examples, refer to U.S. Pat. No. 4,831,389 which shows a multicolor off-board ink supply system; U.S. Pat. No. 4,929,963 which demonstrates an ink delivery system for an ink-jet printer using a low pressure recirculating pumping system; and U.S. Pat. No. 4,968,998, which teaches an ink-jet pen which is refillable at a service station [each assigned to the common assignee of the present invention and incorporated herein by reference.]
In the related application U.S. application Ser. No. 08/065,957, back pressure regulation of an ink-jet pen having an off-board ink reservoir has been demonstrated. In such systems, ink level and an out-of-ink condition sensing is even more critical. Therefore, in a printhead device, such as an ink-jet pen, which uses a replaceable or refillable ink supply, it is desirable to know when the supply is depleted before the printhead is operated in an out-of-ink condition.